1. Field of the Invention
The present invention relates to a premixed air-fuel mixture supply device for supplying a premixed air-fuel mixture to a combustor for a gas turbine or an aircraft engine and, more particularly, to a premixed air-fuel mixture supply device for supplying a premixed air-fuel mixture to a combustor to make the combustor combust a premixed air-fuel mixture in a lean-burn mode, reduce NOx and prevent the deterioration of combustion while the combustor is in a low-load operation.
2. Description of the Related Art
A conventional combustor for a gas turbine or an aircraft engine has a combustor casing, and a cylindrical or annular combustor liner disposed in the combustor casing to define a combustion chamber. A fuel nozzle is connected to a head part of the combustor liner. The combustor casing and the combustor liner define an air passage through which air supplied by an air compressor flows into the combustion chamber.
When fuel is injected in air for diffusive combustion in the combustion chamber of this combustor of a gas turbine or an aircraft engine, high-temperature regions are formed locally in the combustion gas, and the high-temperature regions increases the concentration of NOx in the combustion gas.
Interest in environmental problems has progressively increased in recent years and restrictions on environmental condition have been intensified. The inlet temperature of recent gas turbines and aircraft engines, namely, the outlet temperature of the combustors of gas turbines and aircraft engines, has been raised to improve the thermal efficiency of the gas turbines and aircraft engines. However, the local high-temperature regions in the combustion gas produced by diffusive combustion increase and the concentration of NOx increases accordingly as the outlet temperature of the combustors of gas turbines and such increases. Therefore, measures for reducing NOx is very important.
A gas turbine combustor with a lean premixed, prevaporized combustion system (a prevaporized, premixed air-fuel mixture lean-burn type combustor for a gas turbine) is proposed to reduce the concentration of NOx in the combustion gas. In this gas turbine combustor, fuel is supplied at a substantially fixed rate in a pilot combustion region on the upstream side of a combustion chamber to produce high-temperature combustion gas by stable combustion, a lean air-fuel mixture is burned in a main combustion region below the pilot combustion region for lean-burn combustion that scarcely produces NOx. When a liquid fuel is vaporized beforehand to produce a prevaporized, premixed air-fuel mixture for lean burn.
Referring to FIG. 3 showing a conventional combustor, compressed air supplied by an air compressor, not shown, flows through a space between a combustor casing 1 and a combustor liner 2. When the combustor is a forward flow combustor, air flows in the direction of the blank arrow (→), and the right end, namely, the downstream end, of the combustor casing 1 is closed. When the combustor is a backward flow combustor, air flows in the direction of the arrow (←), and the left end, namely, the downstream end, of the combustor casing is closed. Combustion air reached the combustor head flows into a pilot combustion air passage 3 and a main combustion air passage 4. Although the main combustion air passage 4 shown in FIG. 3 is divided into two air passages 4a and 4b, the main combustion air passage 4 does not necessarily need to be divided.
Referring to FIG. 4 showing a premixing air-fuel mixture supply device, pilot fuel is injected out through fuel injection holes 5a formed in a pilot fuel injection nozzle 5 and arranged at angular intervals. Swirl devices 6a and 6b for swirling combustion air are disposed above the fuel injection holes 5a. Main fuel is injected out through main fuel injection holes 7 arranged at angular intervals. Swirl devices 8a and 8b for swirling combustion air are disposed above the main fuel injection holes 7. An atomization lip 9 extends downstream from the swirl devices 8a and 8b to atomize the main fuel. A prevaporizing, premixing chamber 10 is supplied into a combustion chamber 15 below the premixing air-fuel mixture supply device. The premixed air-fuel mixture burns in the combustion chamber 15.
Related techniques are disclosed in JP-A 8-42851, JP-A 9-145057 and JP-A 2002-206744.
The following problems arise when this previously proposed prevaporized, premixed air-fuel mixture lean-burn type combustor uses both the pilot fuel and the premixed air-fuel mixture while the combustor is in a low-load operation. The fuel injected by the premixed air-fuel mixture supply device is unable to vaporize in the prevaporizing, premixing chamber because the temperature of air around the fuel is comparatively low, unvaporized fuel drops mixed in the swirling air are caused to adhere to a wall defining the prevaporizing, premixing chamber by centrifugal force and the fuel cannot be satisfactorily atomized and vaporized. Consequently, the quality of combustion of the premixed air-fuel mixture in the combustion chamber is deteriorated.
While the prevaporized, premixed air-fuel mixture lean-burn type combustor is in a high-load operation, the quality of combustion in the combustion chamber is not deteriorated because the temperature around the injected fuel is sufficiently high, and fuel droplets are vaporized substantially completely before reaching the wall defining the prevaporizing, premixing chamber.